When a thyristor is turned off, its semiconductor body still contains a relatively high number of free charge carriers which are not dissipated until within a so-called recovery time to the extent where the thyristor finally effects blocking reliably.
However, if the thyristor is subjected to a pulsed voltage loading within the recovery time, then an uncontrolled triggering of the thyristor may occur on account of a still excessively high number of free charge carriers in the semiconductor body. In this case, current filaments may form which have such a high current density that the thyristor may be destroyed.
Therefore, it is generally attempted to construct thyristors in such a way that they are insensitive to voltage surges which occur in the forward direction within the recovery time.
DE 199 47 028 A1 discloses an asymmetrically blocking thyristor with a driver stage. The driver stage comprises an n-doped auxiliary emitter, which makes contact with a metallization of the driver stage. Three variants are provided for improving the recovery protection in this case.
In a first variant, the transistor gain factor of a transistor formed from the n-doped auxiliary emitter, the p-doped base and the n-doped base below the metallization of the driver stage is greater than the transistor gain factor of a transistor formed from the n-doped emitter, the p-doped base and the n-doped base below the cathode of the thyristor.
In a second variant, the transistor gain factor of a transistor formed from the p-doped base, the n-doped base and the p-doped emitter below the metallization of the at least one driver stage is greater than the transistor gain factor of a transistor formed from the p-doped base, the n-doped base and the p-doped emitter below the cathode of the thyristor.
A third variant provides anode short circuits which connect the n-doped base and the anode electrode of the thyristor to one another and the electrical conductivity of which is smaller below the metallization of at least one driver stage than below the cathode electrode.
H. J. Schulze et al.: “Thyristor with Integrated Forward Recovery Protection” Proc. ISPSD 2001 firstly discloses, in the case of a symmetrically blocking thyristor with a triggering stage structure, setting the charge carrier lifetime such that it is significantly higher in the region of the triggering stage structure than in the region of the main cathode.
Secondly, the disclosure of said document shows n-doped islands incorporated into the p-doped emitter. Said n-doped islands act as local transistors which make additional free charge carriers available during the blocking phase of the thyristor.